Following Twitter to mid-Market are cloud-based help desk software vendor Zendesk and dating site Zoosk, who will also benefit from city tax incentives. Storify, maker of a social-media curation tool that launched last year, is among tech companies with offices nearby.

The boom in San Francisco has been influential enough to reverse some long-standing commercial real estate trends. Prices in the SoMa district have risen to meet, and even briefly exceed, those of the more traditional business district on the other side of Market Street, according to TRI Real Estate data. The mid-Market area has drawn so much rental activity that TRI will introduce the neighborhood designation for the first time in its next quarterly report on real estate trends in San Francisco, Phu said.

To be sure, Silicon Valley is still home to many of the biggest names in tech. Oracle, Hewlett-Packard, Intel and Google still have their headquarters there. and are pursuing expansion plans for their Silicon Valley offices.

But with even some established companies, including Cisco Systems and eBay, establishing beachheads in San Francisco, the city's role as a tech heavyweight seems poised to continue. Tech companies want to be where the action is, and, for now, it's in San Francisco.

The IDG News Service